High-performance Bifunctional Catalysts Research Articles

Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media

Spinel NiCo2O4 is considered a promising precious metal-free catalyst that is also carbon-free for oxygen electrocatalysis applications. Current efforts mainly focus on optimal chemical doping and substituent to tune its electronic structures for enhanced activity. Here, we study its morphology control and elucidate the morphology-dependent catalyst performance for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Three types of NiCo2O4 catalysts with significantly different morphologies were prepared using temple-free, Pluronic-123 (P-123) soft, and SiO2 hard templates, respectively, via hydrothermal methods following by a calcination. While the hard-template yields sphere-like dense structures, soft-template assists the formation of a unique nano-needle cluster and flower-like assembly containing abundant meso- and macro pores. Then, the effect of morphology of NiCo2O4 on their corresponding bifunctional catalytic performance was systematically investigated. The flower-like nano-needle assembly NiCo2O4 catalyst via the soft template method exhibited the highest catalytic activity and stability for both ORR and OER. In particular, it exhibited an onset and half-wave potentials of 0.94 and 0.82 V vs. RHE, respectively, for the ORR in alkaline media. Although it is still inferior to Pt, the NiCo2O4 represents one of the best ORR catalyst compared to other reported carbon-free oxides. Meanwhile, remarkable OER activity and stability were achieved with an onset potential of 1.48 V and a current density of 15 mA/cm2 at 1.6 V, showing no activity loss after 20,000 potential cycles (0 to 1.9 V). The demonstrated stability is even superior to Ir for the OER. The morphology-controlled approach provides an effective solution to create a robust 3D architecture with increased surface areas and enhanced mass transfer. Importantly, the soft template can yield high degree of spinel crystallinity with ideal stoichiometric ratios between Ni and Co, thus promoting structural integrity with enhanced electrical conductivity and catalytic properties. The high performance bifunctional oxide catalyst is carbon free and can eventually overcome the stability issue resulting from carbon corrosion in reversible fuel cells and metal-air batteries.

Size-Controlled Carbon Catalysts Derived from Metal-Organic Frameworks for Non-Aqueous Li-Air Battery

The excessive use of fossil fuels has led to increasing concerns about global climate change and national energy storage. It is urgent to develop some clean, efficient, and reliable technologies to harvest and store the clean energy generated from solar and wind. Electrochemical energy technology such as batteries and fuel cells can serve as an ideal medium to store and release electricity. Among various electrochemical systems, Li-air battery has attracted wide attention to be a promising next-generation energy storage device due to its high theoretical capacity (11,000 Wh/kg). Thus, one of the most important topics is to develop an efficient, economical and safe Li-air battery. Here, we develop a series of Co-doped metal-organic frameworks derived carbon catalysts (Co-MOFs) of different sizes, which can be employed as the high-performance bifunctional cathode catalysts for Li-air battery. Controlled by anion type and the amount of 1-methyl imidazole, Co-MOFs catalysts can be obtained with various sizes, ranging from 50 nm to 5 µm. Further optimization of the Co content and thermal activation procedure can produce a highly active Co-N-C catalyst, showing a half-wave potential of 0.818 V vs. RHE in aqueous solution and high selectivity for four-electron reduction, mainly due to the influence of the final morphology and structure including surface area, pore structure and nitrogen doping. Co-N-C catalyst also exhibits a higher performance in terms of onset potential, half-wave potential and current density in non-aqueous solution when compared with non-metal-MOFs derived catalysts. The high oxygen reduction reaction (ORR) activity of this catalyst could be attributed to the Co-Nx active sites embedded in the porous carbon matrix. In the battery, the cathode consisting of Co-MOFs catalysts shows a high discharged capacity of 13,000 mAh/g and a relative stable cycling performance up to 50 cycles, which implies that Co-MOFs catalysts could be a promising candidate as the replacement of platinum group metal catalysts for Li-air battery applications.

Particle-in-box nanostructured materials created via spatially confined pyrolysis as high performance bifunctional catalysts for electrochemical overall water splitting

Novel particle-in-box nanostructure was developed and demonstrated as a high performance bifunctional catalyst for electrochemical overall water splitting. The particle-in-box nanostructure was composed of N-doped graphene layer coated Fe-Ni alloy nanoparticles encapsulated within an N-doped carbon hollow nanobox. The nanobox serves as a nanoreactor with the reactant, water, and catalyst nanoparticles confined within for intimate contact and effective reaction. The particle-in-box nanostructure was created through a spatially confined pyrolysis process by taking polydopamine coated Ni3[Fe(CN)6]2·H2O nanocubes as the precursor, calcination of which leading to formation of an N-doped porous carbon shell from the carbonization of the polydopamine coating layer and formation of interior N-doped graphene layer coated Fe-Ni alloy nanoparticles from reduction of both iron and nickel ions and graphitization of the CN groups. The particle-in-box nanostructured catalyst exhibited excellent electrocatalytic activities, achieving overpotentials of 270 and 201 mV for the oxygen and hydrogen evolution reactions, respectively at 10 mA/cm2, and thus is suitable of serving as a high performance bifunctional catalyst for the overall water splitting reaction, achieving a low full cell overpotential of 471 mV at 10 mA/cm2. The excellent electrocatalytic activity may be attributed to the advantageous particle-in-box nanostructure offering confined and effective reaction environment and the synergistic effects resulting from the conductive, hollow, and porous carbon shell and the small size of the Fe-Ni alloy nanoparticle core and its N-doped graphene layer shell. This new nanostructure of particle-in-box is superior and advantageous for a wide range of applications, not only for electrocatalytical reactions but also for other heterogeneous reaction systems.

Co3O4/MnO2/Hierarchically Porous Carbon as Superior Bifunctional Electrodes for Liquid and All-Solid-State Rechargeable Zinc-Air Batteries.

The design of efficient, durable, and affordable catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is very indispensable in liquid-type and flexible all-solid-state zinc-air batteries. Herein, we present a high-performance bifunctional catalyst with cobalt and manganese oxides supported on porous carbon (Co3O4/MnO2/PQ-7). The optimized Co3O4/MnO2/PQ-7 exhibited a comparable ORR performance with commercial Pt/C and a more superior OER performance than all of the other prepared catalysts, including commercial Pt/C. When applied to practical aqueous (6.0 M KOH) zinc-air batteries, the Co3O4/MnO2/porous carbon hybrid catalysts exhibited exceptional performance, such as a maximum discharge peak power density as high as 257 mW cm-2 and the most stable charge-discharge durability over 50 h with negligible deactivation to date. More importantly, a series of flexible all-solid-state zinc-air batteries can be fabricated by the Co3O4/MnO2/porous carbon with a layer-by-layer method. The optimal catalyst (Co3O4/MnO2/PQ-7) exhibited an excellent peak power density of 45 mW cm-2. The discharge potentials almost remained unchanged for 6 h at 5 mA cm-2 and possessed a long cycle life (2.5 h@5 mA cm-2). These results make the optimized Co3O4/MnO2/PQ-7 a promising cathode candidate for both liquid-type and flexible all-solid-state zinc-air batteries.

Correction: Facile synthesis of silver nanowire-zeolitic imidazolate framework 67 composites as high-performance bifunctional oxygen catalysts.

Correction for 'Facile synthesis of silver nanowire-zeolitic imidazolate framework 67 composites as high-performance bifunctional oxygen catalysts' by Yuyin Wang et al., Nanoscale, 2018, 10, 15755-15762.

Facile synthesis of silver nanowire-zeolitic imidazolate framework 67 composites as high-performance bifunctional oxygen catalysts.

The development of inexpensive, highly active, bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is required for many energy conversion systems, including water splitting, fuel cells, and metal-air batteries. This study reports silver nanowires-zeolitic imidazolate framework (Ag NWs-ZIF67) composites as bifunctional electrocatalysts for OER and ORR. The synergistic effects of Co2+, organic ligands, and Ag NWs enhance the bifunctional electrocatalytic properties. Ag NWs improved the electrical conductivity of the Ag NWs-ZIF67 composite toward OER, and porous ZIF-67 on the exterior of this product enabled O2 to fully react with Ag NWs for the ORR because O2 molecules were easily deflected from a planar electrode surface. This study provides valuable insights into the rational design of bifunctional oxygen catalysts having advantages of high performance and low cost.

Simple conversion of earth-abundant coal to high-performance bifunctional catalysts for reversible oxygen electrodes

Efficient bifunctional electrocatalysts for reversible oxygen electrodes are vitally important for the realization of important future generation electrochemical energy conversion devices, such as regenerative fuel cells and rechargeable metal–air batteries.

Cubic Mn2O3 nanoparticles on carbon as bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

Transition metal oxides are most promising non-platinum catalysts for oxygen reduction (ORR) and oxygen evolution reactions (OER) in renewable-energy technologies. Among transition metal oxides, Mn oxides are very active for both ORR and OER and can act as a bifunctional catalyst. But due to their poor electronic conductivity, carbon materials are used as supports or mixed with other metal oxides to increase their conductivity that assistances in electrochemical applications. Herein, a highly active mesoporous cubic material consisting of Mn2O3 nanoparticles grown on carbon (Vulcan XC-72 R) as a high performance bifunctional catalyst for both ORR and OER have been synthesized. The synthesized Mn2O3/C has been characterized by various structural analyses. The Mn2O3/C material exhibits much better ORR activity compared to the commercially available Pt/C and Pd/C in alkaline media. The same material is also active for OER, making it a bifunctional electrocatalyst for both the reactions. Consequently, Mn2O3/C is quite stable up to 1000 cycles displaying its better stability. The reaction mechanism follows a 4-electron pathway for ORR. The superior electrocatalytic presentation mainly arises due to the better synergistic coupling effect of carbon and Mn2O3.

Coupling cobalt-iron bimetallic nitrides and N-doped multi-walled carbon nanotubes as high-performance bifunctional catalysts for oxygen evolution and reduction reaction

Zn-air battery, as an ideal energy conversion and storage device, is always limited by the expensive and less-than-ideal air electrode materials. The coupling of outstanding oxygen evolution reaction (OER) active sites (oxides derived from Co-Fe nitrides) and superior oxygen reduction reaction (ORR) active centers (metal-N-C and graphitic N) to acquire high-performance and low-cost catalysts is an ideal solution. Herein, we have successfully combined cobalt-iron bimetallic nitrides with N-doped multi-walled carbon nanotubes (Co-Fe-N@MWCNT) as a robust bifunctional material. Benefiting from the synergistic effect between Co, Fe and MWCNTs, Co-Fe-N@MWCNT not only possesses large electrochemically active surface area and effective transport path, but also realizes the integration of superior OER and ORR active sites. Only a low overpotential (290mV) is needed to achieve a current density of 10mA cm-2 for OER and the ORR catalytic activity is close to that of the commercial Pt/C. Additionally, Co-Fe-N@MWCNT as an ideal air electrode material can also be applied in Zn-air battery, which exhibits low voltage drop and favorable stability. The voltage gap has a slight change (about 0.03V) even after 100 cycles of galvanostatic charge-discharge. More importantly, the synthetic strategy in our work may facilitate the design of more high-efficient bifunctional catalysts in various domains.

Nanocarbon/oxide composite catalysts for bifunctional oxygen reduction and evolution in reversible alkaline fuel cells: A mini review

A reversible fuel cell (RFC), which integrates a fuel cell with an electrolyzer, is similar to a rechargeable battery. This technology lies on high-performance bifunctional catalysts for the oxygen reduction reaction (ORR) in the fuel cell mode and the oxygen evolution reaction (OER) in the electrolyzer mode. Current catalysts are platinum group metals (PGM) such as Pt and Ir, which are expensive and scarce. Therefore, it is highly desirable to develop PGM-free catalysts for large-scale application of RFCs. In this mini review, we discussed the most promising nanocarbon/oxide composite catalysts for ORR/OER bifunctional catalysis in alkaline media, which is mainly based on our recent progress. Starting with the effectiveness of selected oxides and nanocarbons in terms of their activity and stability, we outlined synthetic methods and the resulting structures and morphologies of catalysts to provide a correlation between synthesis, structure, and property. A special emphasis is put on understanding of the possible synergistic effect between oxide and nanocarbon for enhanced performance. Finally, a few nanocomposite catalysts are discussed as typical examples to elucidate the rules of designing highly active and durable bifunctional catalysts for RFC applications.

La0.8Sr0.2MnO3-Based Perovskite Nanoparticles with the A-Site Deficiency as High Performance Bifunctional Oxygen Catalyst in Alkaline Solution.

Perovskite (La0.8Sr0.2)1-xMn1-xIrxO3 (x = 0 (LSM) and 0.05 (LSMI)) nanoparticles with particle size of 20-50 nm are prepared by the polymer-assisted chemical solution method and demonstrated as high performance bifunctional oxygen catalyst in alkaline solution. As compared with LSM, LSMI with the A-site deficiency and the B-site iridium (Ir)-doping has a larger lattice, lower valence state of transition metal, and weaker metal-OH bonding; therefore, it increases the concentration of oxygen vacancy and enhances both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). LSMI exhibits superior ORR performance with only 30 mV onset potential difference from the commercial Pt/C catalyst and significant enhancement in electrocatalytic activity in the OER process, resulting in the best oxygen electrode material among all the reported perovskite oxides. LSMI also exhibits high durability for both ORR (only 18 mV negative shift for the half-wave potential compared with the initial ORR) and OER process with 10% decay. The electrochemical results indicate that the A-site deficiency and Ir-doping in perovskite oxides could be promising catalysts for the applications in fuel cells, metal-air batteries, and solar fuel synthesis.

(Invited) Perovskite Oxide As High Performance Bifunctional Catalyst in Alkaline Solution

The global energy crisis coupling with the consumption of fossil fuels and the associated environmental issues, has stimulated extensive interest in searching for clean, efficient and sustainable energy storage and conversion systems. Producing oxygen through an OER process can be promising when effective catalysis of water oxidation into oxygen molecules could be achieved. Therefore, developing an efficient OER electrocatalyst is vital to the new generation of electrochemical storage and conversion devices such as electrolyzer, hydrogen production from water and metal-air batteries. However, there are still several challenges that have to be solved for the electrochemical water oxidation process to become economically attractive One of them is associated with the high overpotential and thereby energy losses at the electrode, where OER is taken place with four electron transfer, according to the following overall reaction: 2H2O (l) + 4 × 1.23 eV → O2 + 4H+ + 4e- (1) Oxygen reduction reaction (ORR), reversible reaction of OER, also play an important role in electrochemical energy storage devices including fuel cell and metal air battery. Advanced ABO3 perovskite oxides with improved catalytic properties have been extensively developed and modified as bifunctional catalyst because of their high versatility in composition, crystalline and electronic structure. In this paper, (La0.8Sr0.2)1-xMn1-xIrxO3 (x=0, 0.05 and 0.1) nanoparticles with particle size of 20-50 nm were succesfully developed as the superior bifuctional catalyst in alkaline solution. The onset potential for ORR is around 0.92V, which is only 50 mv negative shift for state of art Pt/C catalyst while the onset potential for OER is around 1.55 V, which is comparabel with a state of art IrO2 catalyst.

Molybdenum modified nickel phyllosilicates as a high performance bifunctional catalyst for deoxygenation of methyl palmitate to alkanes under mild conditions

A new Mo doping Ni phyllosilicates catalyst (Mo–Ni@PSi) was synthesized and exhibited high catalytic activity under mild reaction condition for the deoxygenation of methyl palmitate.

Cobalt-copper layered double hydroxide nanosheets as high performance bifunctional catalysts for rechargeable lithium-air batteries

Designing novel cost-effective catalysts is one of the greatest challenges in the development of high-performance rechargeable Li-air battery. Here, we report the synthesis of novel cobalt-copper layered double hydroxide (LDH) nanosheets via a simple co-precipitation method under Ar gas. The resulting Co-Cu LDH nanosheets, possessing an average thickness of 4.3 nm and a high surface area of 163 m2 g−1, have been applied as bifunctional catalysts for Li-air battery. In the simulated air environment, Li-air batteries with Co-Cu LDH nanosheets demonstrate higher discharge capacity of 4870 mAh g−1 at 200 mA g−1, lower charge overpotential and better cycling stability with controlling capacity of 1000 mAh g−1, which is much better than that with CuCo2O4 electrode. The enhanced electrochemical performance is ascribed to the large interface between electrolytes and catalysts resulting from high surface area of Co-Cu LDH, providing plenty of actives sites to promote the formation/decomposition of Li2O2. The unique ultrathin nanosheet structure of Co-Cu LDH can also facilitate the mass transportation of O2 and ions in Li-air battery.

Mesoporous carbon derived from vitamin B12: a high-performance bifunctional catalyst for imine formation

Mesoporous carbon derived from natural vitamin B12 is applied for the first time in organic synthesis and exhibits exceptionally high dual activity for imine formation via the cross-coupling of alcohols with amines and the self-coupling of primary amines using molecular oxygen or air as the terminal oxidant.

Cobalt-embedded nitrogen doped carbon nanotubes: a bifunctional catalyst for oxygen electrode reactions in a wide pH range.

Electrocatalysts for the oxygen reduction and evolution reactions (ORR/OER) are often functionally separated, meaning that they are only proficient at one of the tasks. Here we report a high-performance bifunctional catalyst for both ORR and OER in both alkaline and neutral media, which is made of cobalt-embedded nitrogen doped carbon nanotubes. In OER, it shows an overpotential of 200 mV in 0.1 M KOH and 300 mV in neutral media, while the current density reaches 50 mA cm(-2) in alkaline media and 10 mA cm(-2) in neutral media at overpotential of 300 mV. In ORR, it is on par with Pt/C in both alkaline and neutral media in terms of overpotential, but its stability is superior. Further study demonstrated that the high performance can be attributed to the coordination of N to Co and the concomitant structural defects arising from the transformation of cobalt-phthalocyanine precursor.

A Bi2Te3@CoNiMo composite as a high performance bifunctional catalyst for hydrogen and oxygen evolution reactions

Renewable hydrogen generated by using solar energy has attracted increasing interest in energy conversion.

Co3O4nanoparticles grown on N-doped Vulcan carbon as a scalable bifunctional electrocatalyst for rechargeable zinc–air batteries

Balancing the loading ofin situgrown Co3O4nanoparticles with the N-doped Vulcan carbon underneath is essential to produce scalable high-performance bifunctional catalysts of Co3O4/NVC for rechargeable Zn–air batteries.

Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction

Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low costs remains a grand challenge. Here, we report a hybrid material of Co3O4 nanocrystals grown on reduced graphene oxide (GO) as a high-performance bi-functional catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). While Co3O4 or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen-doping of graphene. The Co3O4/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high performance non-precious metal based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co3O4 and graphene.